OBJECTIVE It has been well known that the stress of heavy-resistance exercise has a potent effect for both strength development and muscle fiber hypertrophy. This may be due, at least in part, to exercise induced acute increase in serum anabolic hormone. Therefore, the purpose of this study was to compare the effects of three different rest intervals on IGF-1, cortisol (CO), Creatine Kinase (CK), and blood lactate concentrations. METHODS Hence, ten recreationally strength-trained men (age=20.37 ± 2.44 years, body mass= 65.6 ± 26.70 kg) volunteered as subjects that in 4 separate sessions with 48 h from each other performed 4 different resistance trainings (RT). In the first session, one repetition maximum (1RM) was determined. From second to forth session, subjects performed 4 sets of squat and bench press with 85% of 1RM until exhausted and one of the rest intervals (60, 90 and/or 120 second) was used in a random order between sets. RESULTS It wasn’t observed a significant difference among the three programs in IGF-1 concentration. However, it was observed a significant increment of 3.6 and 23 percent in IGF-1 concentration immediately and 30 min after exercise during RT with 60 s rest between sets. CO concentration in protocols with 60 and 90 s rest increased significantly more than 120 s protocol. CK and blood lactate concentration changes weren’t differing between protocols but post exercise CK concentration significantly increased in each protocol. DISCUSSION & CONCLUSION Growth factors, including IGF-I are known to be mediators of satellite cell activation, increased protein synthesis, decreased protein degradation, hyperplasia, and myofibril hypertrophy during muscle growth and development (Jennische, 1987). The response of IGF-I to acute RT is less clear. In the present study, although postexercise values of IGF-1 during three protocols increased but IGF-1 concentration changes immediately after protocols were significant only in RT program with 60 s rest bet
